Some fantastic thoughts in here, and I think many of us are in good agreement on the overall evolution of the pattern, I think in particular @Isotherm and I have never quite agreed to this distinct level on the likelihood of cold and snow. There is a harmony developing between many global forcing mechanisms and pattern drivers - from the lower levels of the atmosphere all the way into the stratosphere - and we continue to see signs of it in the medium to long range.
Coming in to this winter, the stratosphere was a major uncertainty. We understood that there was a higher likelihood that the vortex would be weaker than normal, but with the QBO in flux it was always a bit of a wild card how the vortex would behave. The state of the vortex as we type today on 12/11/2018 is far beyond where I even thought it would be. It is weak, it is displaced, and it has been continually stretched. Almost all ensembles indicate that the vortex will approach the minimum range of climatological strength over the next two to three weeks, which is when the vortex is typically beginning to approach its climatological peak strength. This is significant in its own right.
The harmonious and continuous Wave 1/Wave 2 hits to the vortex continue, and we are now on our fourth recurring Scandinavian ridge anomaly. The Wave 1 and Wave 2 hits have not quite been at ideal sync yet, but they have done their damage. There is excellent ensemble agreement that the subsequent Wave 1 hits near Christmas will deliver a significant blow to the stratospheric polar vortex, with a major warming event and very high anomalies near the pole. 45-75N heat flux forecasts are into the 10% range on the latest forecasts.
This is not insignificant. The stratosphere is going to become a major influencer on the weather pattern, and high latitude ridging should become increasingly prevalent. We have seen some dissonance from the Pacific over the past two weeks and this is largely expected to continue into the period around 12/20. However, thereafter the agreement builds substantially that the Pacific jet should retract and even maintain significant poleward reach. Tropical forcing and global forcing from Asia aligns during this time as well.
This leads us to a potential "goldilocks zone" from 12/25 onward, give or take a few days. At this time the potential rises notably for unusually cold air to move southward into Canada. There may be a brief lag as Canada had undergone a period of notable moderation in the weeks prior. However, between 12/25 - 1/5 there is increasing likelihood that deep cold will move southward as a larger -EPO ridge builds from British Columbia into the Arctic regions.
It is not lost on me that the Atlantic will be significant as well - and the exact evolution of the stratosphere remains indeterminate at this time. However, it is my opinion that Atlantic high latitude blocking should become increasingly likely as that time frame goes on and we move into the first and second week of January. With a large scale ridge building poleward into Western Canada and Atlantic ridging becoming more notable (particularly into Greenland), the chances of cold and snow in the Eastern United States should begin to increase substantially.
The potential exists for a memorable period of cold and winter weather potential during this general time frame. I am not comfortable forecasting more specific dates at this time, but the harmony of the above features leads me to believe that the likelihood of either a memorable period and/or winter storm(s) comes to a gradual crescendo during the period between 12/25 and 1/15 across the Eastern United States, particularly from the Northern Mid-Atlantic into New England.